In the development of radio communication systems, such as mobile communication systems (like for example GSM (Global System for Mobile Communication), GPRS (General Packet Radio Service), UMTS (Universal Mobile Telecommunication System) or the like), efforts are made for an evolution of the radio access part thereof. In this regard, the evolution of radio access networks (like for example the GSM EDGE radio access network (GERAN) and the Universal Terrestrial Radio Access Network (UTRAN) or the like) is currently addressed. Such improved radio access networks are sometimes denoted as evolved radio access networks (like for example the Evolved Universal Terrestrial Radio Access Network (E-UTRAN)) or as being part of a long-term evolution (LTE) or LTE-Advanced. Although such denominations primarily stem from 3GPP (Third Generation Partnership Project) terminology, the usage thereof hereinafter does not limit the respective description to 3GPP technology, but generally refers to any kind of radio access evolution irrespective of the underlying system architecture. Another example for an applicable broadband access system may for example be IEEE 802.16 also known as WiMAX (Worldwide Interoperability for Microwave Access).
In the following, for the sake of intelligibility, LTE (Long-Term Evolution according to 3GPP terminology) or LTE-Advanced is taken as a non-limiting example for a broadband radio access network being applicable in the context of the present invention and its embodiments. However, it is to be noted that any kind of radio access network may likewise be applicable, as long as it exhibits comparable features and characteristics as described hereinafter.
In the development of cellular systems in general, and access networks in particular, relaying has been proposed as one concept. In relaying, a terminal or user equipment (UE) is not directly connected with an access node such as a radio base station (e.g. denoted as eNodeB or eNB) of a radio access network (RAN), but via a relay node (RN). Relaying by way of relay nodes has been proposed as a concept for coverage extension in cellular systems. Apart from this main goal of coverage extension, introducing relay concepts can also help in providing high-bit-rate coverage in high shadowing environments, reducing the average radio-transmission power at the a user equipment (thereby leading to long battery life), enhancing cell capacity and effective throughput, (e.g. increasing cell-edge capacity and balancing cell load), and enhancing overall performance and deployment cost of radio access networks.
FIG. 1 shows a schematic diagram of a typical deployment scenario of a relay-enhanced access network, such as e.g. a LTE or LTE-Advanced RAN with radio-relayed extensions. As shown in FIG. 1, UEs at disadvantaged positions such as a cell edge and/or high shadowing areas are connected to a so-called donor base station (DeNB) via a respective relay node RN. Generally, any one of the relay nodes may be stationary/fixed or mobile. The link between the DeNB and RN may be referred to as backhaul link/connection (while, generally, a backhaul link/connection may be regarded to be any link between a base station and a node towards a core network side, e.g. a link between a micro/macro/pico/femto home base station and the core network), relay link or Un link, and the link between RN and UE may be referred to as access link or Uu link.
As, in the deployment scenario according to FIG. 1, a link between the DeNB and a core network (not shown) may also be referred to as a backhaul link/connection, such relay-enhanced cellular system may be said to contain several types of backhaul connections.
Recently, the concept of relaying is adopted in the context of LTE-Advanced.
In the context of LTE-Advanced, a Layer 3 (L3) RN, also referred to as Type I RN or self-backhauling RN, is currently taken as a baseline case for the study on relay extensions. Such a relay node, which is exemplarily assumed for the further description, appears as a normal base station towards its served terminals or user equipments (UE). That is, such relay node is required to have all the essential release 8 eNB cell parameters and to broadcast them so that it could be recognized as a normal eNB cell by the UEs.
Accordingly, both the DeNB as well as the RNs may be regarded as base stations of an access network, possibly as base stations of different hierarchical level in terms of logical and/or structural network deployment.
However, the concept of relaying, i.e. the introduction of relay nodes, also leads to several problems. In the present description, the resulting increase of end-to-end delay in relay-enhanced connections is particularly considered.
Such increase of the end-to-end delay is basically due to the fact that data has to be forwarded between DeNBs and RNs, increasing the delay as compared with a scenario of macro eNBs (such as DeNBs) only. The extra delay is even more severe than it looks from a mere counting of the additional hops. This is because these hops will not necessarily be performed sequentially in a time-wise manner without some interruption, which is due to the usage of the Multicast Broadcast Single Frequency Network (MBSFN) frame structure for the communication between RN and DeNB. This is because LTE UEs (e.g. LTE release 8 UEs) are expected to monitor the Physical Downlink Control Channel (PDCCH) for Reference Signals (RSs) all the time, unless they are under Discontinuous Reception (DRX) sleep mode, and as such it is not feasible to switch off the link between the RN and its UEs when the RN-DeNB link is active. In the MBSFN solution, the Orthogonal Frequency Division Multiplexing (OFDM) symbols that are specified (e.g. in LTE release 8) for MBSFN are used to switch the RN into reception mode from the DeNB, while the UEs (e.g. LTE release 8 UEs) will assume that this is some MBSFN transmission with low power and, thus, will not make any use of the signals transmitted there.
A further increase of the end-to-end delay is caused by handovers in a relay-enhanced access network, in particular handovers of UEs between two relay nodes controlled by different DeNBs.
FIG. 2 shows a schematic diagram of a handover scenario in a deployment scenario of a relay-enhanced access network with radio-relayed extensions. As shown in FIG. 2, a user equipment UE is connected to its serving base station denoted as source donor base station (DeNB) via a relay node denoted as source relay node (RN). When a handover of the UE to another cell is performed, as indicated by the dashed arrow in FIG. 2, the user equipment will then be connected to its new serving base station denoted as target donor base station (DeNB) via a new relay node denoted as target relay node (RN). The individual connections being indicated by double-sided double-line arrows may be any kind of physical and/or logical connection, including for example X2 interface connections between relay nodes and base stations or between base stations.
FIG. 3 shows a signalling diagram of a handover preparation procedure, in particular an admission control for handover preparation, in the handover scenario according to FIG. 2.
In such a case, assuming that, for example, 1/10th of the sub-frames are allocated for the backhaul link between RN and DeNB, the additional (relaying-caused) handover request and handover request ACK messages between the target RN and DeNB may lead to at least 10 ms and up to 20 ms of additional delay in the handover process. It might also be required to have a resource reconfiguration in the backhaul link during the backhaul admission control process, when there are resource limitations. That is, the DeNB may reconfigure the resource partitioning so that more sub-frames are allocated for the backhaul or the access links. This process will also cause additional delay in the handover process, as the UE is required to wait while the resource re-partitioning is being performed.
Any delay, in particular additional end-to-end delay due to relaying, is particularly adverse for delay-sensitive bearers. Namely, real-time and/or delay-sensitive active bearers of a UE subject to a handover process may thus experience severe quality degradation.
Moreover, the end-to-end delay is even further increased in a multi-hopping case in which multiple RNs being controlled by the same DeNB may be connected to each other in a chain-like manner so that multiple hops may be needed from a RN to its DeNB. That is, in such scenario, the UE data has to pass via several RNs before it reaches the UE (in case of downlink) or the DeNB (in case of uplink), respectively. The multi-hopping nature of such relaying scenarios increases the overall end-to-end delay of packets, and as such it might not be appropriate for certain bearers or services, in particular real-time and/or delay-sensitive active bearers or services.
Another concept in the context of LTE and LTE-Advanced is the concept of Home NodeBs (HNBs) and Home eNodeBs (HeNBs), also known as femto cells.
The specification work for Home NodeB (HNBs) and Home eNodeB (HeNBs), also known as femto cells, is applicable for LTE release 8 and beyond. A HNB or HeNB is basically a small base station that uses an alternative backhaul connection to the mobile core network (which in case of HNB may be a direct link to the core network, and in case of HeNB may be a link via a HeNB gateway towards the core network), such as the subscriber's fixed DSL (digital subscriber line) internet connection, instead of the usual microwave or high capacity leased or fibre optics lines that usually connect base stations to the core network. This is especially advantageous in many aspects such as better indoor coverage and load balancing where the subscribers inside a given household or office building will be served via the HNB, thereby freeing the macro cells for other users.
In such femto cell environment, a link between the HNB/HeNB and the core network may be said to be a backhaul connection of different type as compared with a link between a normal/macro base station and the core network.
However, the end-to-end delay in a HNB/HeNB case may end up being higher and more unpredictable than in a normal macro/micro/pico base station case, because part of the path is through the Internet, or another operator's DSL network, where the operator might have no control over, and the available bandwidth therein may have to be shared with the traffic from multiple users, becoming liable to congestion. Thus, as in the relaying case discussed above, HNBs/HeNBs might not be appropriate for serving certain bearers or services, in particular real-time and/or delay-sensitive active bearers or services.
That is to say, similar problems and drawbacks may prevail both in a relay cell environment as well as a femto cell environment.
In view thereof, there exist various problems in the context of handover processes in access networks comprising several types of backhaul connections between base stations, such as especially in relay-enhanced access networks or access networks with femto cells. Especially when UEs are intended to be handed over (from a DeNB or RN) to a RN or (from a macro/micro/pico home base station or a HNB/HeNB) to a HNB/HeNB, conventional handover processes on the basis of measurement reports of the UE may be insufficient or at least inefficient (e.g. in terms of delay conditions).
In order to prevent a UE from handing over to a specific cell, such as a relay cell or a femto cell, the eNB or DeNB or macro/micro/pico base station can ignore situations that would have normally led to a handover to the concerned cell (for example, if the measurement reports from the UE indicate that the signal strength of the concerned cell has been satisfying the handover criteria). This can be done, for example, by modifying the neighbour relation table (NRT) such that the “no HO” (HO: handover) flag is checked for the concerned cell, or by way of access class barring, where the concerned cell could be marked as “reserved” or “barred”.
Yet, both above-mentioned approaches are cell-specific. That is, such ignoring of a handover demand would apply to all UEs in the entire cell being marked or barred accordingly. This is undesirable, since the situation may be different for different UEs with respect to the same cell.
In view thereof, it is noted that preventing handovers of UEs to a specific cell, such as a relay cell or a femto cell, may not be appropriate with any one of the above-outlined approaches. That is, the above-outlined approaches are not capable of properly addressing the foregoing problems in the context of handover processes in access networks with several types of backhaul connections, such as especially in relay-enhanced access networks or access networks with femto cells (e.g. in terms of delay conditions).
Accordingly, a UE-specific approach for ignoring handover situations is preferable. This can be accomplished, for example, by the eNB making a note that a certain UE should not be handed over to certain cell or cells (e.g. by way of a UE-specific NRT), or by the eNB instructing the UE to put certain cells in its “black list” and to not send measurement reports for such cells according to the Radio Resource Control (RRC) or Medium Access Control (MAC) specifications.
Currently, no such feasible mechanism exists for facilitating efficient handover control in networks with several types of backhaul connections, such as for example in relay-enhanced access networks or access networks with femto cells. Such problem generally exists for any backhaul connections in any kind of access network, wherein backhaul connections between donor base stations and relay nodes in relay-enhanced access networks may be referred to as a specific, yet non-limiting, example in this regard as well as, for example, (backhaul) connections between femto cells/home base stations and the core network in access networks that contain both macro and femto cells.
Accordingly, there is a demand for mechanisms for facilitating efficient handover control for backhaul connections.